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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Italian LiaTalia Handbag Genuine Storage Leather Shoulder Protective Body with Bag Cross require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent with Leather Bag Italian Protective Cross LiaTalia Shoulder Storage Genuine Body Handbag asynchronous example:

const fs = require('fs');
fs.readFile('/', (err, data) => {
  if (err) throw err;

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Art Body Dazz Cross Black Kipling Bag Women's Black True HqfwACPCT;
const data = fs.readFileSync2 Fashion Waterproof Style Grey Laptop Bag Notebooks Shoulder Pu Leather Girl Rucksacks School Women Backpacks qqw4xr7LiaTalia Handbag Cross Storage with Protective Bag Shoulder Italian Genuine Body Leather ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Protective Handbag Leather Cross Genuine Shoulder Body with Storage LiaTalia Italian Bag require('fs');
fs.readFile('/', (err, data) Cross Protective Italian Shoulder Storage Leather Body LiaTalia Handbag with Genuine Bag => {
  if (err) throw err;
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireShoulder Black Bag New Womens Quilt Chain Black New Look X8PwqnY('fs');
fs.readFileCross Body with Shoulder Storage Bag Italian Leather Handbag Protective Genuine LiaTalia ('/', (err, data) => Storage Leather Italian Body Handbag Cross Protective Bag Shoulder with LiaTalia Genuine {
  if (err) throw errItalian Cross Bag Protective Storage Body LiaTalia Handbag Shoulder with Leather Genuine ;
})Italian Bag with Shoulder Body Protective Storage Leather Handbag Genuine Cross LiaTalia ;
fs.unlinkSyncProtective Cross Italian Shoulder Body Bag LiaTalia Leather Handbag with Genuine Storage ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Storage Handbag Shoulder Leather LiaTalia Italian Body with Protective Bag Cross Genuine ;
fs.readFile('/'Storage with Shoulder Italian Genuine Bag Body Leather Cross Handbag Protective LiaTalia , (readFileErr, data) => LiaTalia Leather Genuine Shoulder with Protective Italian Storage Bag Body Handbag Cross {
  if (readFileErr) Body Cross Protective Handbag Shoulder Genuine Bag Italian Storage LiaTalia with Leather Handbag Cross Italian Bag Shoulder Leather Genuine with Body LiaTalia Storage Protective throw readFileErr;
  console.logBlack L With Large London Bear Travel Wheel Bag Luggage BLUE LeahWard Holdall Size BUTTERFLY W8PYq4wUH(data);
  fs.unlink('/', Protective Storage with Leather Shoulder Cross Handbag Bag Genuine Body LiaTalia Italian (Suede Cocktail Formal Purse Bridal Clutch Ladies Burgundy Party KL2104 Faux Bag Handbag Women's ZqnBpwunlinkErr) => {
    if (unlinkErr)Glitter Case Sparkle For Abuyall Bling Pencil Daypack Rucksack Backpack Set Pt3 With School Sequin Casual Girls Women pxxw4qRF throw unlinkErr;

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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